Oocyte-Granulosa Cell Signaling in 4-Vinylcyclohexene Diepoxide-Induced Ovotoxicity

Abstract

At birth, the mammalian ovary has a finite number of dormant primordial follicles. Repeated daily dosing of rats with the occupational chemical, 4- vinylcyclohexene diepoxide (VCD), depletes the ovary of small pre-antral follicles (primordial and primary follicles) through an increase in the natural process of atresia (apoptosis). In addition, in vitro exposure of postnatal day 4 (PND4) rat ovaries to VCD causes a similar depletion of ovarian follicles. Since many growth factors play crucial roles in the promotion of early folliculogenesis and follicle survival, it is possible that any number of factors and subsequent signaling pathways could be disrupted in response to VCD exposure. Therefore, the studies in this work address the hypothesis that VCD disrupts oocyte-granulosa cell survival pathways in the rat ovary, thereby compromising cell-cell communication and causing follicle cell death. The results from the first aim reveal that through the use of genomic analyses a subset of genes were determined to be affected via in vivo and in vitro exposure routes to VCD. The results of the second aim show that two transforming growth factor β (TGFβ) growth factors, growth and differentiation factor-9 (GDF-9), and bone morphogenetic factor-4 (BMP-4), are not likely involved in VCD-induced ovotoxicity as they were unable to prevent ovarian follicle loss in the presence of VCD. The results of the third aim reveal that expression of the c-Kit receptor, present on the oocytes, is decreased and its ligand, Kit Ligand (KL), produced from the granulosa cells, is increased in response to in vitro VCD exposure. In addition, attenuation of VCD-induced follicle loss occurs in the presence of exogenous KL. Finally, the results of the fourth aim examines the involvement of the AKT signaling molecule in response to VCD exposure, in which the active phosphorylated AKT is determined to be down-regulated by VCD. Taken together, these studies show that VCD is able to disrupt at least one of the cellular survival pathways that are crucial to maintain the ovarian follicle. As a result, a breakdown in cell-cell communication may occur at that level and contribute to an increase in follicular atresia and eventual cell death.